The present invention relates to a method and a device for recording or reproducing information to or from recording media by using holography.
Optical discs with a recording capacity of about 50 GB that conform to the Blu-ray Disc™ standard based on blue-violet semiconductor laser diodes have now become available also for consumer use. In the future, there will be a growing need to increase the optical disc capacity to as large as 100 GB to 1 TB, almost the same level as that of hard disk drives (HDD).
To realize such an ultrahigh density with optical discs, however, requires a new high density recording technology, different from the conventional one that involves shortening a wavelength of light and increasing a numerical aperture (NA) of an objective lens.
With studies on next generation storage technology currently under way, a hologram recording technology that records digital information by using holography is attracting attention.
The hologram recording technology records information in a recording media by overlapping a signal beam, which has information on page data modulated two-dimensionally by a spatial light modulator, on a reference beam in the interior of the recording media and causing an interference pattern produced by the beam overlapping operation to modulate a refractive index in the recording media.
For reproduction of information, the reference beam that was used for recording is thrown onto the recorded media to cause the hologram recorded in the media to work as a diffraction grating, producing a diffracted beam. This diffracted beam is reproduced as the same light as the recorded signal beam combined with phase information.
The reproduced signal beam is detected two-dimensionally at high speed by a light detector such as CMOS and CCD. As described above, the hologram recording technology allows two-dimensional information to be recorded in optical recording media at one time and then reproduced using a single hologram. Further, since a plurality of pieces of page data can be written one layer over the other at a desired location in the recording media, a large-capacity, high-speed recording and reproduction of information is assured.
In this hologram recording technology, US Publication 2006/0192866, the counterpart JP Publication of which is JP-A-2006-236536, describes a method that can deal with a situation where a spatial light modulator or a light detector in the recording/reproducing device has defects or where defects are produced as a result of use of the device. This patent literature describes a recording/reproducing device which “comprises: a reference beam emitting unit to shine a reference beam onto a hologram recording medium to read page data recorded in the hologram recording medium; a spatial light modulation unit to generate an information beam that is modulated by using spatial information corresponding to the page data to be written into the hologram recording medium; an information beam emitting unit to shine the information beam to the same area that is shone by the reference beam; a light detection unit to receive a reproduced light generated by the reference beam shone on the hologram recording medium; a known data storage unit to store known page data in advance; a defect examination unit to check defect positions on the spatial light modulation unit or light detection unit, first by writing the known page data in the hologram recording medium by shining on the hologram recording medium the information beam, which has been modulated by using the spatial information corresponding to the known page data, and the reference beam and second by reading the written known page data from a reproduced light that the light detection unit receives from the hologram recording medium by shining the reference beam on the medium; and a reproduction control unit to restore original data on defect positions, which is included in the reproduced light that the light detection unit has received, by using the information on the defect positions detected by the defect examination unit; wherein same data D4 as the known page data D3 stored in the known data storage unit is recorded in the hologram recording medium in advance; wherein a first defect position examination and a second defect position examination are executed, the first examination being done by the defect examination unit recording the known page data D3 in the hologram recording medium and then reading the recorded data D3′, the second examination being done by the defect examination unit reading the data D4 recorded in advance in the hologram recording medium; wherein a decision unit is provided which, based on the information about the defect positions detected by the two defect position examinations, checks which of the spatial light modulation unit and the light detection unit has defects, so that the data on the defect positions can be restored by using the defect position information detected by the defect examination unit.” “Even if the spatial light modulation unit or light detection unit has defects or if defects result through the use of the device, this recording/reproducing device therefore can correctly record or reproduce the defect position data in or from the hologram recording medium as long as the number of defects is within an allowable range. This recording/reproducing device also offers an improved yield, reduced production cost and high reliability in recording and reproducing user data.”